Hitherto, in a working vehicle such as a dump truck, the influence of the load on each part of a vehicle, such as the body, the tires, the wheel mounting portions, and the suspension is increased by the weight of earth, sand, stones, and rocks which are loaded on the vehicle at the time of traveling. Particularly, when the load weight is heavier than the set maximum allowable load weight, i.e., when so-called overloaded, the influence of the overload on each part of the vehicle is remarkably increased in a state where a transmission is shifted to a maximum speed stage and the vehicle runs at the maximum engine speed, i.e., a state where the vehicle runs at its allowable maximum speed. For this reason, not only the durability of each part of the vehicle, the tire life, and the ride quality are deteriorated while traveling, but also a road surface is likely to be damaged, so it is necessary to repair the road surface frequently.
In order to solve the above problem, an apparatus for limiting the vehicle speed is proposed so that the vehicle speed does not increase to an allowable maximum speed corresponding to the load weight or higher during traveling under the control of an accelerator pedal (hereinafter, referred to as accelerated traveling). As such an apparatus, a vehicle speed limiting apparatus is proposed in Japanese Unexamined Patent Publication No. 5-221251, for example, in which the shifting of a transmission to a higher speed stage is prohibited in order to limit the maximum traveling speed of a dump truck.
A conventional vehicle speed limiting apparatus will be described with reference to FIGS. 20 to 23. Referring to FIG. 20, the steering front wheels 82 are mounted to the left and right of the front portion of a vehicle body 81 through a suspension cylinder 83, and the driving rear wheels 84 are mounted to the left and right of the back portion of the vehicle body 81 through a suspension cylinder 85. A driver's cab 86 is mounted on the front portion of the vehicle body 81, and a cargo container 87 is vertically pivotably mounted on the back of the vehicle body 81 via a hoist cylinder 88. Each of the suspension cylinders 83 and 85 is provided with a pressure sensor 89 for detecting the pressure in the respective cylinder extension chamber.
Referring to FIG. 21, an output shaft of an engine 45 is connected to an input shaft of a transmission 91, such as a torque flow transmission, and an output shaft of the transmission 91 is connected to the driving rear wheels 84. A speed change control valve 94, for changing each speed change stage, is attached to the transmission 91, and each speed change stage of the transmission 91 is switched by actuating the speed change control valve 94 with a shift signal from a speed change controller 93. A load weight calculator 99 receives pressure signals from the pressure sensors 89 of the suspension cylinders 83 and 85, calculates the load weight from the pressure signals, and outputs a calculated load weight signal to the speed change controller 93. A speed stage signal, from a shift lever 95 located in the driver's cab 86, is inputted to the speed change controller 93.
FIG. 22 shows a case where the speed of the engine 45, having a mechanical governor device, is mechanically controlled by an accelerator pedal 1. That is, when the accelerator pedal 1 is depressed to a position at an angle .theta.op, a member 143 rotates about a first central axis 142 by an angle proportional to the angle .theta.op, whereby the member 143 rotates an accelerator lever 144 of the engine 45 to form an angle corresponding to the angle .theta.op. The accelerator lever 144 actuates a fuel injection pump (not shown) through a mechanical governor device (not shown) integrated into the engine 45, and controls the fuel injection amount and the fuel injection timing so as to obtain an engine speed corresponding to the rotation angle of the accelerator lever 144.
Assume that a characteristic diagram of a fuel injection amount control by the above mechanical governor device is as shown in FIG. 23, for example. The fuel injection amount abruptly decreases at the time when the engine speed increases so as to exceed a maximum allowable speed Nmax, so that the maximum speed of the engine is controlled at any position of the accelerator lever 144.
An operation of the above vehicle speed limiting device (maximum speed limiting device) will now be described.
In the extension chamber of the suspension cylinders 83 and 85, a pressure, which is consistent with the weight of the vehicle body 81 and the cargo container 87, is generated during an empty load; and a pressure, which is higher than the pressure during the empty load by the pressure consistent with the load weight of the cargo container, is generated when earth, sand, etc., are loaded. The load weight calculator 99 obtains a load weight based on the difference between a pressure signal from the pressure sensors 89 when loading earth, sand, etc., and a pressure signal during an empty load, and outputs a load weight signal to the speed change controller 93.
A speed stage suited to the load weight is stored in the speed change controller 93, and a suitable speed stage is selected by a load weight signal from the load weight calculator 99. For example, when the calculated load weight is lighter than the maximum allowable load weight, the speed stage is not limited, and the speed stage can be shifted to the maximum speed stage of the vehicle. Therefore, when the shift lever 95 is switched to the sixth forward speed in a vehicle having six stages of forward speed, the speed change controller 93 outputs to the speed change control valve 94 a shift signal for the sixth forward speed stage, whereby the transmission 91 is switched to the sixth forward speed stage. The traveling speed of the vehicle at this time is controlled to a maximum speed (for example, 45.5 km/h), which is equivalent to the maximum allowable engine speed at the sixth forward speed stage.
However, when the calculated load weight is heavier than the maximum allowable load weight, the upper limit speed stage is limited by the overload weight, and a shifting from the upper limit speed stage to a further upper stage is prohibited. For example, at the time of an overload such that the load weight is 130% of the maximum allowable load weight, the upper limit speed stage is limited to the fifth speed stage during forward motion. Therefore, even if the shift lever 95 is switched to the sixth forward speed stage in the same manner as described above, the speed change controller 93 outputs to the speed change control valve 94 a shift signal for the fifth forward speed stage, whereby the transmission 91 is switched to the fifth forward speed stage.
At this time, even if the accelerator pedal 1 is depressed so as to increase the speed to the maximum speed or higher, the engine speed is limited by the mechanical governor device to the maximum allowable speed or lower. As a result, the traveling speed is limited to the maximum speed at the speed stage thereof. In this way, the traveling speed is controlled to the maximum speed (for example, 33.5 km/h), which is equivalent to the maximum allowable engine speed in the fifth forward speed stage, or lower.
However, according to the vehicle speed limit by the above mechanical governor device, when the engine speed increases to the maximum allowable speed Nmax or higher, the fuel injection amount to the engine 45 abruptly decreases, so that the engine output torque also abruptly decreases. For this reason, when traveling on an uphill grade of large traveling resistance, or on a road surface having many irregularities, the engine rotational speed decreases considerably. However, with the decrease in the rotational speed, the fuel injection amount again increases to increase the engine output torque, so that the vehicle is accelerated again. In this way, when the engine speed is the maximum allowable speed Nmax, the vehicle speed tends to decrease or increase abruptly. This phenomenon might further increase the influence of the load on each part of the vehicle to deteriorate the durability of the vehicle and the ride quality during driving.
In addition, a working vehicle, such as a dump truck, is driven in a work field in cooperation with an excavator, etc., and a plurality of dump trucks and excavators are usually systematically operated within a wide field of a construction area. In such a case, the vehicle speed limit value of each dump truck is preferably set to a value suited to the weather conditions, the field environment, and the system operation conditions. For example, it is necessary to reduce the vehicle speed limit value on a slippery road surface on a bad weather day, and to set the vehicle speed limit value suited to the overload weight in response to road irregularities condition. In addition, it is necessary to set the vehicle speed limit value to a safe operational speed on an operation course where a plurality of dump tracks frequently come and go.
However, the vehicle speed limit value is fixed to the speed determined by the maximum allowable speed Nmax and the speed change gear ratio at each speed stage. That is, the vehicle speed limit value cannot be set to the value suited to the weather conditions, the field environment, and the system operation conditions. For this reason, a driver of the dump truck must drive such that the vehicle speed does not exceed the allowable speed while sufficiently considering the various environmental conditions and operation conditions, thus increasing fatigue during the driving operation.
Another known vehicle speed limiting apparatus is an apparatus for limiting the engine speed to the maximum allowable speed or lower even if the accelerator pedal is depressed to offer the maximum speed or higher, i.e., a so-called vehicle speed limiting apparatus by a rotary limiter. This vehicle speed limiting apparatus, as in the case of the vehicle speed limit by the above mechanical governor device, limits the upper limit speed stage corresponding to an overload weight during overloading, so that the maximum speed at each speed stage is limited to the maximum speed determined by the maximum allowable speed Nmax and the speed change gear ratio for the respective speed stage.
However, according to the vehicle speed limit by the rotary limiter, the fuel supply to the engine is forcibly stopped when the engine rotational speed increases to the maximum allowable speed Nmax, and the fuel supply is started again when the engine rotational speed decreases by a predetermined value with respect to the maximum allowable speed Nmax. For this reason, as in the case of the above mechanical governor, when traveling on an uphill grade, etc., the engine rotational speed decreases considerably, and there is a tendency to cause an abrupt increase or decrease in the vehicle speed. It is thought that such a tendency to torque starvation becomes serious, particularly when the engine maximum allowable speed Nmax is set near a maximum torque point. This phenomenon might further increase the influence of the load on each part of the vehicle to deteriorate the durability of the vehicle and the ride quality during driving.